1. Field of the Invention
The invention relates to the areas of piezoelectricity and magnetism, and relates in particular to piezoelectric or magnetic materials and a method for manufacturing the same.
2. Description of Related Art
In the area of piezoelectricity and magnetism, the use of composite materials including a matrix and particles within the matrix is known.
Such composite materials can be used in certain devices such as magnetometers, compasses, or magnetic direction finders.
The magnetic composites currently made employ particles or grains of magnetic substances derived from materials such as ferrites, e.g., magnetic iron oxide (FeO) (Fe.sub.2 O.sub.3), natural ferrite made of Fe.sup.2+ and Fe.sup.3+ ions, or other ferrites based on Co, Cu, Mn, Mg, or Li associated with Fe.sup.2+ and Fe.sup.3+ ions in the oxide form.
These grains of magnetic substance have one major drawback: their sizes and shapes are fairly irregular, because they are obtained by grinding operations, possibly with additional screening operations. The grinding and screening operations very substantially increase the cost of manufacturing the grains. Moreover, high-quality magnetic compounds must have a high level of physical and chemical homogeneity, which is not necessarily ensured by the manufacturing process described.
A piezoelectric composite material can be used in sensors of the electromechanical transducer type. These transducers can be used in hydrophones, accelerometers, hydrostatic pressure sensors, deformation, force, or temperature sensors, or complex systems that can combine these functions. These transducers can also be used in air, microphones, teletype or computer keyboards, heat detectors, or piezochrome coatings.
One drawback to the piezoelectric materials containing piezoelectric particles and described in the prior art resides in the fact that the piezoelectric particles have sizes greater than one micron, with the size distribution ranging from a few microns to several hundred microns. As with magnetic composite materials, one condition for obtaining a high-quality piezoelectric composite material is regularity of particle size and shape, as explained in U.S. Pat. No. 4,407,054 for example.
French Patent Application FR9413743 published under number 2719181 partially overcomes this drawback. French Patent Application FR9413743 describes variable-polarization piezosensitive devices using a piezosensitive material composed of a) piezoelectric particles and a matrix of either a polymer or a dielectric organic compound, wherein the organic compound is either non-piezoelectric or slightly piezoelectric, or b) an agglomerate of particles.
French Patent Application FR9413743 also describes several methods for manufacturing these particles depending on a desired size of the particles.
For example, one can use a guillotine to cut particles from sheets, plates or wires of piezoelectric plastics. The piezoelectric plastics can be prepolarized in the thickness direction.
Also, when monocrystals of mineral origin such as brushite or Seignette salt (i.e., sodium potassium tartrate) are synthesized, the crystallization process can be speeded up by shaking the solution using vibration or ultrasound. Crystallides of far smaller size than the particles generally obtained by ordinary methods are formed in this manner.
Also, when biological materials which themselves have piezoelectric properties are used, these materials can be divided by differential ultracentrifucration; this technique yields particles with an approximate size of 5 nanometers (nm).
However, the size and shape of the particles obtained by these methods still have a certain dispersion inherent in the manufacturing method.